John Gill

And he that shall swear by heaven,.... As the Jews were wont to do in common, but did not look upon such an oath as obligatory on them; See Gill on Mat 5:34, though such an one sweareth by the throne of God; for heaven is God's throne, where he sits, and, in an eminent manner, displays the glory of his majesty: and by him that sitteth thereon, by God himself. Thus swearing by anything that has any relation to God, is implicitly swearing by him; and therefore ought to be considered as binding, as if he was expressed in it; since an appeal cannot be made to things inanimate, nor indeed to any creature, but to God, the searcher of hearts.